Simple, Rapid Chemical Labeling and Screening of Antibodies with Luminescent Peptides

Development of Luminescent Biosensors for Calprotectin

Calprotectin, a metal ion-binding protein complex, plays a crucial role in the innate immune system and has gained prominence as a biomarker for various intestinal and systemic inflammatory and infectious diseases, including inflammatory bowel disease (IBD) and tuberculosis (TB). Current clinical testing methods rely on enzyme-linked immunosorbent assays (ELISAs), limiting accessibility and convenience. In this study, we introduce the Fab-Enabled Split-luciferase Calprotectin Assay (FESCA), a novel quantitative method for calprotectin measurement. FESCA utilizes two new fragment antigen binding proteins (Fabs), CP16 and CP17, that bind to different epitopes of the calprotectin complex. These Fabs are fused with split NanoLuc luciferase fragments, enabling the reconstitution of active luciferase upon binding to calprotectin either in solution or in varied immobilized assay formats. FESCA's output luminescence can be measured with standard laboratory equipment as well as consumer-grade cell phone cameras. FESCA can detect physiologically relevant calprotectin levels across various sample types, including serum, plasma, and whole blood. Notably, FESCA can detect abnormally elevated native calprotectin from TB patients. In summary, FESCA presents a convenient, low-cost, and quantitative method for assessing calprotectin levels in various biological samples, with the potential to improve the diagnosis and monitoring of inflammatory diseases, especially in at-home or point-of-care settings.

Split-Protein Therapeutic Platforms: Identifying Binder Pairs

Therapeutic proteins, including enzymes, interferons, interleukins, and growth factors, are emerging as important modalities to treat many diseases that elude management by small molecule drugs. One challenge of protein treatment is the propensity for off-target or systemic activity. A promising approach to overcome such toxicity is to create conditionally active constructs by splitting the therapeutic protein into two, or more, inactive fragments and by fusing these fragments to binders (e.g., antibodies) that target distinct epitopes on a cell surface. When these antibodies bind to their respective targets, the protein fragments are brought into proximity and then reconstitute into the active form of the therapeutic protein. In this chapter, we describe approaches to determine antibody pairs that enable the reconstitution of the active protein. General computational and empirical methods are provided to facilitate the identification of pairs starting only from protein sequence data.

Mechanical Unfolding and Refolding of NanoLuc via Single-Molecule Force Spectroscopy and Computer Simulations

A highly bioluminescent protein, NanoLuc (Nluc), has seen numerous applications in biological assays since its creation. We recently engineered a NanoLuc polyprotein that showed high bioluminescence but displayed a strong misfolding propensity after mechanical unfolding. Here, we present our single-molecule force spectroscopy (SMFS) studies by atomic force microscopy (AFM) and steered molecular dynamics (SMD) simulations on two new hybrid protein constructs comprised of Nluc and I91 titin domains, I91-I91-Nluc-I91-I91-I91-I91 (I91₂-Nluc-I91₄) and I91-Nluc-I91-Nluc-I91-Nluc-I91, to characterize the unfolding behavior of Nluc in detail and to further investigate its misfolding properties that we observed earlier for the I91₂-Nluc₃-I91₂ construct. Our SMFS results confirm that Nluc's unfolding proceeds similarly in all constructs; however, Nluc's refolding differs in these constructs, and its misfolding is minimized when Nluc is monomeric or separated by I91 domains. Our simulations on monomeric Nluc, Nluc dyads, and Nluc triads pinpointed the origin of its mechanical stability and captured interesting unfolding intermediates, which we also observed experimentally.

Development of a rapid, simple, and sensitive point-of-care technology platform utilizing ternary NanoLuc

Point-of-care tests are highly valuable in providing fast results for medical decisions for greater flexibility in patient care. Many diagnostic tests, such as ELISAs, that are commonly used within clinical laboratory settings require trained technicians, laborious workflows, and complex instrumentation hindering their translation into point-of-care applications. Herein, we demonstrate the use of a homogeneous, bioluminescent-based, split reporter platform that enables a simple, sensitive, and rapid method for analyte detection in clinical samples. We developed this point-of-care application using an optimized ternary, split-NanoLuc luciferase reporter system that consists of two small reporter peptides added as appendages to analyte-specific affinity reagents. A bright, stable bioluminescent signal is generated as the affinity reagents bind to the analyte, allowing for proximity-induced complementation between the two reporter peptides and the polypeptide protein, in addition to the furimazine substrate. Through lyophilization of the stabilized reporter system with the formulated substrate, we demonstrate a shelf-stable, all-in-one, add-and-read analyte-detection system for use in complex sample matrices at the point-of-care. We highlight the modularity of this platform using two distinct SARS-CoV-2 model systems: SARS-CoV-2 N-antigen detection for active infections and anti-SARS-CoV-2 antibodies for immunity status detection using chemically conjugated or genetically fused affinity reagents, respectively. This technology provides a simple and standardized method to develop rapid, robust, and sensitive analyte-detection assays with flexible assay formatting making this an ideal platform for research, clinical laboratory, as well as point-of-care applications utilizing a simple handheld luminometer.